1. Field of the Invention
The current invention relates to a battery operated portable electronic device having dual batteries, and more particularly to a portable battery operated device storing data for retrieval be a user of the device. The invention also relates to battery systems for portable electronic device.
2. Background Information
As the size of portable electronic devices decreases and the complexity of the tasks required of them increases significant pressure is put on battery life. Battery technology has improved greatly over the years with storage capacities increasing and size reducing. However, much of the disadvantages associated with battery life are inherent in the operating requirements of modern electronic devices. Traditional analogue devices draw a steady current, however modern digital devices can load a battery with short, high current spikes. The current a battery can deliver is dependent on its state of charge and internal resistance. In many instances although a battery still has stored energy available it is unable to supply this energy in the way required by modern portable device.
A good example of this problem can be found in modern portable music players. Many consumers now demand a device with very high media storage capacity. Due to the high cost of large capacity solid-state storage mediums many manufacturers use small form hard disk drives (HDDs) in their device. HDDs consume significant amounts of power. To increase battery life a small solid-state memory module is also included to which blocks of data are copied from the HDD so that the HDD need only operate intermittently. This small solid-state memory module might hold, say, 4 minutes worth of playback data so that every 4 minutes the HDD must be turned on for a very short period to copy a new block of data to the solid-state memory module. The normal operating requirements of the device is typically only a few tens of milliamps but rises to hundreds of milliamps during operation of the HDD.
The remaining charge in the device battery is typically determined by measuring the battery output voltage. However, as battery state of charge decreases output voltage drops significantly at high operating currents and so this method becomes unreliable due to the periodic and variable operation of the HDD. Therefore, to ensure proper shutdown of the device this must occur when the measured battery state of charge indicates at least 10% charge left. If it were not for the periodic and variable high currents required by the HDD shutdown would not need to occur until the battery state of charge reached 1%-2%.
FIG. 1 shows the discharge characteristic of a typical Lithium Ion battery to illustrate this problem. The voltage at 90% capacity drops from 3.5V to 3.0V when the current drawn varies from 0.5 C to 2 C. In comparison at 90% capacity the voltage is close to 3.7V when the current drawn is less than 0.2 C.
Another disadvantage with this type of device is that the periodic and variable high currents required by the HDD prevent the use of cheaper non-rechargeable carbon-zinc or zinc-chloride batteries (sometime called Leclanché cells) due to their large internal resistance. More costly alkaline batteries can be used, but battery life will be short due to the short, high current nature of operation.